The long-term goal is to employ a multidisciplinary approachto gain a thorough understanding of how the GTPase cycle is utilized in various biological systems to control specifically diverse cellular functions, including signal transduction, cytoskeleton organization, and intracellular membrane trafficking. The current research project focuses on the functional specificity of Rab GTPases with the RabS interaction with its effector RabaptinS as the structural basis. The Rab GTPase family contains about 40 distinct members (60 if isoforms are included) in the human genome and each interacts with a set of specific effectors and promotes a specific membrane fusion or transport function in the membrane trafficking system. Thus an important question is how each Rab distinguishes from other Rabs in recognizing its specific effectors. This functional specificity issue remains to be resolved at molecular and structural levels, which should have a broad impact not only on the GTPase field but also on the protein-protein recognition mechanisms in general. In this project, the mechanism of Rab5-Rabaptin5 interaction and its role in endosome fusion and membrane trafficking will be investigated, based on our recent Rab5-Rabaptin5 crystal structure. In addition to RabaptinS, other RabS effectors in fusion (EEA1 and RabenosynS) will also be investigated and compared in the experiments below. Aim 1 is to define the interaction interface of RabS-RabaptinS complex and elucidate the specificity of this Rab-effector interaction via mutagenesis, biochemical binding assays and structural studies. Aim 2 is to test the Rab specificity by converting another Rab into functional RabS through structure-based engineering. Aim 3 addresses the mechanism of RabaptinSfunction in early endosome fusion. A new mechanism involving RabaptinS tetramerization is proposed based on the crystal structure and will be tested in functional assays. Aim 4 addresses the role of RabaptinS in the biogenesis and maintenance of RabS- and Rab4-positive endosomes during endocytosis via expressing its RabS- or Rab4- binding domain or Rab5/Rab4-binding defective fragments in the cell to disrupt specifically the endogenous RabaptinS interaction with RabS or Rab4. This project should contribute to our understanding of a variety of diseases involving membrane trafficking and Rab GTPases, including heart diseases, cancers, skin and retinal diseases.